Fluid actuators such as expansible motors utilizing cylinders and reciprocal pistons defining chambers alternately pressurized and exhausted of fluid medium commonly use cushioning means to control deceleration of the piston movement as the piston approaches the cylinder head. Cushioning permits large pressurized fluid medium volumes and pressures to be utilized to produce rapid piston movements and high force capacities without imposing damage upon the piston structure due to hammering or impact of the piston structure with the actuator heads at the termination of piston movement in a given direction. Cushioning apparatus lengthens the fluid actuator life, reduces the noise attendant with actuator operation and increases the working life of the components being driven by the actuator.
The most common fluid actuator piston cushion structure utilizes a fluid port defined in the actuator head coaxial with the piston axis. This port communicates with the source of fluid pressure, and the fluid exhaust conduit or reservoir, selectively, through appropriate valve devices. The cushioning apparatus normally comprises a valve member mounted upon the piston structure coaxial with the piston axis and the valve member either circumscribes the piston rod, on the rod end of the piston, or constitutes a projection or button on the other side of the piston coaxial with the rod axis. It is usually desired that cushioning occur at the termination of piston movement in either direction with double acting expansible motors, and in most expansible motors valve members are utilized on both piston sides.
Valve members mounted upon piston structure for cushioning purposes are telescopingly received within the ports defined within cylinder heads through which the pressurized medium is introduced or exhausted from the cylinder chamber. Seal rings and the like are often utilized within the head port for cooperating with the valve member, such as shown in U.S. Pat. Nos. 2,493,602 and 2,704,996. Also, it is known to form the cushion sealing means in such a manner, or support the seal ring in such a manner, as to permit the seal to be self aligning with respect to the piston mounted valve member as shown in the assignee's U.S. Pat. Nos. 2,719,510 and Re. 24,532.
The presence of the valve member within the head port restricts the flow of pressurized medium through that port when it is desired to pressurize the adjacent cylinder chamber and it is common to utilize axially movable valve member seal rings and bypass passages permitting fluid to flow around the seal ring and valve member when pressurizing the cylinder chamber as shown in U.S. Pat. Nos. 2,853,974, 2,935,047 and 3,267,815.
As the fluid medium is being exhausted from the cylinder chamber forward of the piston movement the entrance of the "leading" valve member into its aligned head port rapidly restricts the rate of fluid flow being exhausted to achieve the desired cushioning and piston movement deceleration, and the valve member seal ring port structure will operate to close bypass passages used during pressurization, and control the rate at which the fluid medium is exhausted during the final stages of piston movement. The rate of piston movement during the final stages of cushioning may be controlled by adjustable needle valves as shown in U.S. Pat. Nos. 2,704,996, 2,719,510 and Re. 24,532, or the fluid may be metered through grooves or slots defined in the valve structure itself, as shown in U.S. Pat. Nos. 3,008,454 and 3,704,650, and in some fluid actuator constructions a plurality of radial orifices defined within tubular valve members progressively decrease the rate that the fluid medium may be exhausted as the piston approaches the adjacent head, as shown in U.S. Pat. Nos. 2,443,312, 3,677,141 and 3,974,910.
Valve member seal rings located within head ports are usually of an elastic material and subject to wear. As fluid actuators are often expected to cycle several million times during their effective life the wear occuring between the valve members and the seal rings gradually permit the exhausting fluid medium to increasingly escape between the surfaces of the valve member and associated seal ring, and as the fluid medium pressures during cushioning may reach very high values leakage due to wear can become significant, and the efficiency of the cushioning apparatus with known cushioning constructions rather rapidly deteriorates. Because of wear effective cushioning over long periods of time, particularly at high fluid pressures and with speed actuators, cannot be maintained and prior art devices have not effectively solved the problem of deterioriating piston cushioning characteristics.
It is an object of the invention to provide cushioning structure for fluid actuators wherein the efficiency of piston cushioning is maintained over long periods of time and through many cycles of operation and wherein consistent cushioning characteristics are maintained despite wide fluid temperature variations.
An additional object of the invention is to provide wear compensating piston cushioning means which is usable with a variety of cushion embodiments, and wherein the improved cushioning structure is of economical manufacture and does not require expensive modifications to existing apparatus, and may be utilized with conventional forms of fluid actuators.
In the practice of the invention the cushioning structure includes a valve member, or members, mounted upon a fluid actuator piston reciprocal within a cylinder enclosed at its ends by heads. The valve members have cylindrical exterior surfaces and are adapted to be telescopically received within bores or ports defined in the cylinder heads. The head ports communicate with passages and conduits wherein pressurized fluid medium may be introduced into the cylinder chambers via the ports, and fluid medium is also exhausted from the cylinder chambers through the ports by the moving piston during a stroke. Valve and control structure, well known in the art, determines the fluid medium circuit exterior of the fluid actuator.
The port includes an annular seal receiving recess or chamber concentric to the piston axis and adjacent the interior head surface. The seal chamber includes an annular seal ring having an outer diameter less than the diameter of the chamber whereby limited radial displacement of the seal ring is possible to permit the seal ring to be radially self-aligning with the associated valve member. Seal ring retaining means, such as a snap ring, are located within the seal ring chamber to axially restrain ring movement toward the piston chamber, and an annular radial shoulder or abutment defines the outermost axial dimension of the seal ring chamber and abuttingly engages a side of the seal ring during cushioning. The axial dimension between the abutment shoulder and snap ring is greater than the axial dimension of the seal ring whereby limited axial movement of the seal ring occurs during pressurization and exhaust cycles. Fluid medium bypass passages are defined in the head and communicate with the seal ring chamber and the cylinder chamber whereby fluid may flow around the seal ring during pressurization of the cylinder chamber through the port.
The seal ring is preferably formed of cast iron and has excellent wear resistant characteristics. The seal ring is split, i.e., is severed or broken at one location through its radial dimension wherein the circumference of the ring may be expanded. The normal I.D. of the inner surface of the seal ring is slightly less than the O.D. of the outer surface of the associated valve member, and the valve member is provided with ring expanding surfaces, whereby entrance of the valve member into the seal ring I.D. slightly expands the seal ring to provide an effective tight sealed relationship, yet permits the relative axial movement between the valve member and ring required. In practice, the seal ring is initially manufactured with an I.D. which would have an interference fit with the associated cushion member. The seal ring is then radially fractured and installed, and the larger diameter of the valve member will cause the seal ring to slightly expand each time it is received therein.
The dimensional relationship between the seal ring and valve member permits the seal ring to automatically compensate for wear occuring between the valve member and seal ring, and an effective sealing relationship will exist between the ring inner surface and valve member outer surface for an extended time. As wear occurs the degree of expansion of the seal ring will slowly decrease and the frictional engagement between the seal ring and valve member will remain substantially constant.
The contractable seal ring described above can be utilized with cushion systems employing bypass and needle valve bleeding systems, and the sealing ring may also be employed with the valve members having slots or grooves defined in the outer surface for fluid flow purposes. Also, the contracting seal ring may be employed with valve members having radial orifices communicating with an internal passage and the advantages thereof will be utilized with all of the aforementioned cushion systems.